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A block comment in Rust is a lexical construct used to instruct the compiler to ignore a specific sequence of characters during the lexical analysis phase. It is delimited by /* at the beginning and */ at the end, allowing the enclosed text to span multiple lines or be embedded directly within a single statement. Standard Syntax The lexer strips the comment and treats everything between the opening and closing delimiters as whitespace before parsing begins. 
/*
   This text is ignored by the Rust compiler.
   It can span multiple lines safely.
*/
Inline Evaluation Because the Rust lexer evaluates block comments as whitespace, they can be placed inline within expressions without breaking the abstract syntax tree (AST). 
let total = 100 + /* offset */ 50;
Nesting Mechanics Unlike C and C++, Rust’s lexical analyzer explicitly supports nested block comments. The compiler maintains a depth counter for the delimiters; every opening /* found within an existing block comment requires a corresponding closing */ before the outermost comment is considered terminated. This prevents premature termination when commenting out blocks of code that already contain block comments. 
/* Level 1 block comment begins
    /* Level 2 nested comment begins
       Level 2 nested comment ends */
Level 1 block comment ends */
Block Documentation Comments Rust differentiates standard block comments from block documentation comments via a specific character immediately following the opening delimiter. Instead of being entirely ignored, the compiler parses these into #[doc] attributes.
  • Outer block doc comment (/** ... */): Applies to the item immediately following it. Parses as #[doc = "..."].
  • Inner block doc comment (/*! ... */): Applies to the enclosing item (such as a module, crate, or function). Parses as #![doc = "..."].
/**
 * This is an outer block documentation comment.
 * It is parsed into a doc attribute for the function below.
 */
pub fn execute() {
    /*!
     * This is an inner block documentation comment.
     * It applies to the enclosing item (the `execute` function itself),
     * appending this text to the function's overall documentation.
     */
}
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